Process for making carbon foam induced by process depressurization

ABSTRACT

A process for producing molded pitch based foam is shown which provides a more uniform density gradient throughout the ultimate product. The process utilizes a pressure drop during processing in order to induce foaming. By inducing foaming through process depressurization, additional viscosity manipulation can be achieved as well as improved density gradient characteristics in the ultimate product.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a process for the manufacture ofpitch foams, to the subsequent conversion of pitch foam to carbon andgraphite foam and to improvements in the manufacturing process toenhance the properties of the end products.

[0003] 2. Description of the Prior Art

[0004] This invention deals with carbon in its various forms and,particularly to carbon “foams”. Carbon foams are a relatively recentarea of commercial interest, although carbon fibers have been usedcommercially in industry for many years. Carbon fibers are known toexhibit extra ordinary mechanical properties due to the unique graphiticmorphology of the extruded filaments. Advanced structural compositeshave been created which advantageously exploit these properties bycreating a disconnected network of graphitic filaments held together byan appropriate matrix. Pitch based carbon foams can be considered suchan interconnected network of ligaments or struts. As such, pitch basedcarbon foams represent a potential alternative as a reinforcement instructural composite materials.

[0005] Additionally, current applications of carbon fibers have evolvedfrom such structural reinforcement applications to thermal or heat sinkapplications. For example, heat sinks have been utilized in theaerospace industry to absorb energy in applications such as missiles andaircraft where rapid heat generation is found. A material with a largespecific heat capacity is placed in contact with the object that isbeing heated. During the heating process, heat is transferred to theheat sink from the hot object and, as the heat sink's temperature rises,it “stores” the heat more rapidly than can be dissipated to theenvironment through normal convections. Carbon foams have beenconsidered for use as such heat sink materials.

[0006] These and other applications have stimulated research into novelreinforcements and composite processing methods for carbon foams. Highthermal conductivity, low weight and low coefficient of thermalexpansion are of primary concern in such designs. For thermal managementapplications, certain designs which have been considered includedsandwich type approaches in which a low density structural corematerial, such as a honeycomb or foam, is sandwiched between a highthermal conductivity face sheet. Structural cores of these type aregenerally limited to low density materials to insure that the weightlimits are not exceeded. At the present time, carbon foams and carbonhoneycomb materials have generally been the only available materials foruse in high temperature applications (exceeding 60° C.). High thermalconductivity carbon honeycomb materials have been extremely expensive tomanufacture, however, as compared to low conductivity honeycombmaterials. Attempts have been made to overcome these shortcomingsthrough the production of pitch based carbon foam materials.

[0007] Typical prior art foaming processes utilized a “blowing”technique to produce a foam of the pitch precursor. The pitch is meltedand pressurized, and then the pressure is reduced. Thermodynamically,this produces a “Flash,” thereby causing the low molecular weightcompounds in the pitch to vaporize (the pitch boils), resulting in apitch foam. See Hagar, Joseph W. and Max L. Lake, “Novel HybridComposites Based on Carbon Foams,” Mat. Res. Soc. Symp., MaterialsResearch Society, 270:29-34 (1992), Hagar, Joseph W. and Max L. Lake,“Formulation of a Mathematical Process Model Process Model for theFoaming of a Mesophase Carbon Precursor,” Mat Res. Soc. Symp., MaterialsResearch Society, 270:35-40 (1992), Gibson, L. J. and M. F. Ashby,Cellular Solids: Structures and Properties, Pergamon Press, New York(1988), Gibson, L. J., Mat Sci and Eng A110, 1 (1989), Knippenberg andB. Lersmacher, Phillips Tech. Rev., 36 a (4), (1976), and Bonzom, A., P.Crepaur and E. J. Moutard, U.S. Pat. No. 4,276,246, (1981). Additivescan be added to promote, or catalyze, the foaming, such as dissolvedgases (like carbon dioxide, or nitrogen) , talc powder, freons, or otherstandard blowing agents used in making polymer foams.

[0008] Then, unlike polymer foams, the pitch based foam must generallybe oxidatively stabilized by heating in air (or oxygen) for many hours,thereby, cross-linking the structure and “setting” the pitch so it doesnot melt, and deform the structure, during carbonization. See Hagar,Joseph W. and Max L. Lake, “Formulation for Mathematical Process Modelfor the Foaming of a Mesophase Carbon Precursor, ” Mat. Res. Soc. Symp.,Materials Research Society, 270:35-40 (1992) and White, J. L., and P. M.Shaeffer, Carbon, 27:697 (1989). This is a time consuming step and canbe an expensive step depending on the part size and equipment required.

[0009] Next, the “set” or oxidized pitch foam is then carbonized in aninert atmosphere to temperatures as high as 1100° C. Then, a final heattreatment can be performed at temperatures as high as 3000° C. to fullyconvert the structure to carbon and produce a carbon foam suitable forstructural reinforcement. The previously described prior art processesresulted in foams which exhibited low thermal conductivities, however.

[0010] Other techniques may utilize a polymeric precursor, such as aphenolic, urethane, or blends of these with pitch. See Hagar, Joseph W.and Max L. Lake, “Idealized Strut Geometries for Open-Celled Foams, ”Mat. Res. Soc. Symp., Materials Research Society, 270:41-46 (1992),Aubert, J. W., (MRS Symposium Proceedings, 207:117-127 (1990), CowlardF. C. and J. C. Lewis, J. of Mat. Sci., 2:507-512 (1967) and Noda, T.,Inagaki and S. Yamada, J. of Non-Crystalline Solids, 1:285-302, (1969).However, these precursors produce a “glassy” or vitreous carbon whichdoes not exhibit graphitic structure and, thus, has a very low thermalconductivity and low stiffness as well. See, Hagar, Joseph W. and Max L.Lake, “Idealized Strut Geometries for Open-Celled Foams, ” Mat. Res.Soc. Symp., Materials Research Society, 270:41-46 (1992).

[0011] An improvement to the previously prescribed prior art techniquesis described in now issued U.S. Pat. No. 6,033,506, issued Mar. 7, 2000to Klett and in issued U.S. Pat. No. 6,037,032, issued Mar. 14, 2000, toKlett et al. The process described in these later patents is less timeconsuming than the techniques previously described, thereby loweringproduction and fabrication costs. Perhaps more importantly, the Klettprocess is unique in providing carbon foams with high thermalconductivities, generally greater than 58 W/mK.

[0012] Although the Klett process was an improvement in pitch basedcarbon foaming processes, the Klett process utilized a static pressureduring the formation of the green artifact (billet). Routinely, thisstatic pressure selected was about 1000 psig. Graphite artifacts made inthis manner have shown a significant density gradient, generally rangingfrom about 0.25 g/cc at the top of a production billet to about 0.60g/cc at the bottom of the billet. Such variations can be undesirable,depending upon the particular end application.

[0013] A need exists, therefore, for further improvements in pitch basedcarbon foams and products produced therefrom in which density gradientsare reduced.

[0014] A need also exists for such a carbon foam exhibiting reducedpore/bubble sizes within the foam during processing.

[0015] A need exists for such a process which prevents or reducesthermally induced stresses in the final product.

[0016] A need also exists for an improved process for producing a pitchbased carbon foam which allows the foam to set faster and which providesan improved ability to manipulate the viscosity of the material duringthe process stage in which the material is in the liquid/foaming state.

SUMMARY OF THE INVENTION

[0017] It is an object of the present invention to provide a pitch basedcarbon foam having a more uniform density gradient profile, with reducedshrinkage and with less tendency to crack as a finished product.

[0018] Density variations in currently produced products are thought tooccur between the foaming and solidification steps of the process whilethe foamed pitch is still in the liquid state. The liquid pitch tends tomigrate due to gravity, thereby making the bottom of the productionbillet denser than the top portion of the billet. The present inventionhas as one object to slow or stop this migration, thereby improving thedensity uniformity of the ultimately produced product.

[0019] By heating the pitch under an increased pressure, the processtemperature can exceed the normal foaming point of the pitch without thepitch actually foaming, i.e., the thermal foaming point is raised.Holding the pitch at such a selected temperature allows the growth ofmesophase domains within the pitch, thereby increasing the pitch'sviscosity. Higher viscosities at this point in the process reduce thepreviously described migration problems. The higher pressure of, forexample, 8000 psig can then be reduced to, for example, 1000 psig,allowing the heat-treated pitch to foam. Thereafter, a small increase intemperature will solidify the foam. Increasing the viscosity of thepitch also reduces the pore/bubble size of the foam. By manipulating thefinal process pressure, greater control over pore size is maintained. Bychanging the hold times and temperatures along with various upper andlower pressure limits, a wider variety of foam products can be produced.

[0020] In a specifically preferred process of the invention forproducing carbon foam, a pitch precursor is introduced to an appropriatelevel within a mold. The pitch has a characteristic boiling or foamingpoint at a given pressure and for a given temperature. Air is purgedfrom the mold and the pitch is pressurized to a preselected initialpressure which will be greater than the ultimate or final pressureutilized. The preselected initial pressure serves to increase theboiling or foaming point of the pitch above the foaming point at thefinal pressure. The pitch is heated to a temperature below thesolidification point but above the liquid and foaming point whichtypically occurs at the final pressure. The pitch is then depressurizedfrom the initial pressure to the final pressure while maintaining theprocess temperature above the typical boiling or foaming temperature atthe final pressure. The foamed pitch is then heated to a temperaturethat solidifies the foamed pitch. The solidified foam pitch can then becooled to room temperature while allowing natural depressurizationduring cooling to thereby produce a carbon foam.

[0021] Additional objects, features and advantages will be apparent inthe written description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a pitch foaming characterization profile of the priorart technique using a static pressure of approximately 1000 psig duringformation of the green artifact; and

[0023]FIG. 2 is a similar pitch foaming characterization profiledemonstrating the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The process of the invention can perhaps best be described withreference to a description of the prior art process as described in nowissued U.S. Pat. No. 6,033,506, issued Mar. 7, 2000 to Klett, entitled“Process for Making Carbon Foam”, and with reference to FIG. 1 of thedrawings. The prior art process used temperature alone to induce foamingwhereas the inventive process uses both temperature and a pressure dropduring processing to induce foaming. With reference to FIG. 1, thepresent invention is directed particularly toward that the portion ofthe process which is illustrated between the point at which the pitchbegins to foam and the point at which the foam begins to solidify.

[0025] In the process described in the '506 Klett patent, a pitch basedfoam is produced by placing pitch powder, granules or pellets into acontainer with the desired final shape of the foam. The pitch can beselected from among the mesophase pitches, isotropic pitches andmixtures thereof. The mesophase pitches include, for example, syntheticpitches, coal based pitches and petroleum based pitches. These pitchmaterials can be solvated if desired. The pitches can be introduced inthe granulated, powdered or pelletized form. One example precursormaterial is the Mitsubishi ARA-24 mesophase pitch. A proper mold releaseagent or film can be applied to the sides of the mold to allow removalof the part. Preferably, a suitable liner, such as an aluminum foilliner is utilized. If the mold is made from pure aluminum, typically nomold release agent is necessary since the molten pitch does not wet thealuminum and thus does not stick to the mold.

[0026] The pitch is then heated in a substantially oxygen-freeenvironment to avoid oxidation of the pitch materials during heating.Preferably, the pitch is heated in a “HIP” furnace which has beenevacuated to less than one torr. Alternatively, the pitch can be heatedunder a blanket of inert gas, such as nitrogen, to avoid oxidation ofthe pitch. The pitch is heated to a temperature approximately 50 to 100°C. above the softening point. For example, where Mitsubishi ARA-24mesophase pitch is used, a temperature of 300° C. is sufficient.

[0027] Once the pitch is melted, if it is heated in a vacuum, the vacuumis released to a nitrogen blanket. The pressure inside the furnace isthen increased up to about 1000 psi and the temperature of the system isthen raised to cause the evolution of pyrolysis gases to form theviscous pitch foam. This viscous pitch foam is fluid and will flow atthis point. However, the viscosity of the foam is dependent on thetemperature and, in general, as the temperature is increased, theviscosity will decease, making it more flowable. The particular foamingtemperature selected is dependent on the precursor pitch to some extentand, in the case illustrated in FIG. 1, the pitch begins to foam atabout 400° C.

[0028] The temperature of the system is then raised to about 800° C., orto a temperature sufficient to coke the pitch (about 475-500° C.) . Thisis performed at a rate of no greater than about 5° C./min and preferablyabout 2° C./min. The temperature is held for at least 15 minutes toachieve an assured soak and then the furnace power is turned off andcooled to room temperature. Preferably, the foam was cooled at a rate ofapproximately 1.5° C./min with release of pressure at a rate ofapproximately 2 psi/min. During the cooling cycle, pressure is releasedgradually to atmospheric conditions. The molded, pitch derived foam isthen separated from the mold.

[0029] The cast pitch derived foam can be post heat treated totemperatures above 2000° C. for conversion to graphitic structure,depending upon the pitch precursor. In general, mesophase pitch isgraphitized significantly easier than isotropic pitches (coal derived orpetroleum derived). The more graphitic the material, the higher thethermal conductivity of the resulting graphitic foam.

[0030] In the specific prior art process illustrated in FIG. 1, theprocess steps were as follows:

[0031] 1. An appropriate mold is selected.

[0032] 2. If needed, a mold release agent or film is applied to thesides of the mold.

[0033] 3. The mold is filled with a pitch in the form of powder,granules or pellets.

[0034] 4. The pitch is heated to 300° C. while maintaining a vacuum ofless than 1 Torr.

[0035] 5. Vacuum is released and the pitch is pressurized to 1000 psiwith nitrogen gas.

[0036] 6. The pitch is heated to a coking temperature between 500-800°C. at a rate of 2.0° C. per minute.

[0037] 7. The foam is held at the predetermined coking temperature for15 minutes.

[0038] 8. The foam is cooled to room temperature at a rate ofapproximately 2.0° C. per minute while simultaneously depressurizing ata rate of approximately 2 psi per minute.

[0039] The prior art process thus utilizes a static pressure, which inthis case is 1000 psi. That is, the pitch is held at a constant pressureof 1000 psi between the pressurization step indicated as (5) above, andthe depressurization step, indicated as (8) above. When billets producedby the above process were analyzed, they were found to contain voids aswell as density gradients.

[0040] The present invention generally tracks the prior art process withthe exception of the pitch foaming to pitch solidification stages. Thespecific steps followed in the process of the invention are listed belowin order to contrast the process steps with the previously describedsteps of the prior art process:

[0041] 1. A mold is filled with a pellet or powder form of mesophasepitch.

[0042] 2. The pitch is then desiccated to assist in removing anyresidual moisture.

[0043] 3. The pitch is placed into a Hot Isostatic Press (HIP).

[0044] 4. The press is purged of air.

[0045] 5. The pitch is pressurized to a high initial pressure, whichincreases the boiling or foaming point higher than the foaming point atthe final pressure.

[0046] 6. The pitch is heated to a temperature below the solidificationpoint but above the liquid and foaming point which typically occurs ifprocessed at the final pressure.

[0047] 7. The pitch is depressurized from the initial pressure to thefinal pressure while maintaining the process temperature above thetypical boiling or foaming temperature at the final pressure.

[0048] 8. The foam is heated to a temperature that solidifies the foamedpitch.

[0049] 9. The foam is cooled to room temperature while allowing naturaldepressurization during cooling.

[0050] 10. The foam is depressurized of any remaining pressure toatmospheric pressure.

[0051] 11. The foam is removed from the HIP and mold.

[0052] The process of the invention thus heats the pitch under an“increased pressure” which, in effect increases the thermal foamingpoint. That is, the process temperatures can exceed the “normal” foamingpoint of the pitch without the pitch actually foaming.

[0053] In the prior art example process, the “normal” process pressureselected was 1000 psi, the pressure being essentially held static duringthe process steps. In the inventive process, the HIP was pressurized to8000 psi as the “increased pressure.” This increased pressure will beunderstood to be an arbitrary number. In other words, assuming that adoubling of pressure generally increases boiling point by about 10° C.,one could roughly estimate the foaming temperature to be affected asfollows: 1000 psi 425° C. 2000 psi 435° C. 4000 psi 445° C. 8000 psi455° C.

[0054] Thus, 8000 psi was selected as the “increased” pressure toachieve the desired pressurization induced foaming effect.

[0055] With reference to FIG. 2 of the drawings, the following examplesare intended to be illustrative of the process steps of the inventionwithout being limiting:

Example I

[0056] 1. The mold is filled with a predetermined amount of pitch togive an appropriate foam height.

[0057] 2. The mold is placed in the HIP.

[0058] 3. The HIP vessel is evacuated to <2 Torr with the vacuum beingheld for 15 minutes.

[0059] 4. The vessel is pressurized to 8000 psi with nitrogen gas.

[0060] 5. The pitch is heated from room temperature to 300° C. at a rateof 3.5° C. per minute.

[0061] 6. The pitch is held at 300° C. for 1 hour.

[0062] 7. The pitch is heated from 300 to 450° C. at a rate of 2.0° perminute.

[0063] 8. The pitch is held at 450° C. for 1 hour. (This increases theviscosity of the liquid pitch).

[0064] 9. The vessel is depressurized from 8000 to 1000 psi at a rate of700 psi per minute while maintaining a temperature of 450° C. (At thisstage, the liquid pitch begins to foam.)

[0065] 10. The foam is heated from 450 to 475° C. at a rate of 2.0° C.per minute.

[0066] 11. The foam is held at 475° C. for 1 hour. (This is the point atwhich the foam begins to set).

[0067] 12. The foam is heated from 475 to 500° C. at a rate of 0.5° C.per minute.

[0068] 13. The foam is held at 500° C. for 30 minutes. (At this point,the foam fully solidifies).

[0069] 14. The foam is heated from 500 to 600° C. at a rate of 1.0° perminute.

[0070] 15. The foam is held at 600° C. for 2 hours.

[0071] 16. The foam is cooled from 600° C. to RT at a rate of 2.0° C.per minute.

[0072] 17. The vessel naturally depressurized during cooldown from 600°C. to room temperature.

[0073] 18. The remaining vessel pressure is released at a rateapproximately 30 psi per minute.

[0074] 19. The foam is removed from the HIP and its mold.

[0075] 20. The foam is then heat treated to approximately 1000° C. forcarbonization.

[0076] 21. The foam is then heat treated to aproximately 2800° C. forgraphitization.

[0077] Results: Thermal Conductivity ranged from 90-132 W/mK.

[0078] Apparent Density ranged from 0.44-0.46 g/cm³.

Example II

[0079] Same as Example I except:

[0080] 8. The pitch is held at 450° C. for 2 hours.

[0081] Results: Thermal Conductivity ranged from 200-244 W/mK.

[0082] Apparent Density ranged from 0.56-0.62 g/cm³.

Example III

[0083] Same as Example I except:

[0084] 9. The vessel is depressurized from 8000 to 1000 psi at a rate of117 psi per minute while maintaining a temperature of 450° C.

[0085] Results: Thermal Conductivity ranged from 146-184 W/mK.

[0086] Apparent Density ranged from 0.43-0.45 g/cm³.

[0087] An invention has been provided with several advantages. Theprocess of the invention results in graphite foams having more uniformdensity gradient properties. The increased viscosity of the pitch duringthe processing operation reduces pore/bubble sizes within the foam.Manipulation of the final process temperature allows greater controlover pore size. Additionally, changing the hold times and temperaturealong with the various upper and lower pressure limits allows theproduction of a wider variety of foam products. The pitch based foams ofthe invention exhibit thermal conductivities ranging from about 90-244W/mK. The process provides a foam which can be set quicker since thetemperature difference between the foaming point and solidification isreduced. Because the pitch is still liquid up to approximately 465° C.,a more effective means of manipulating the viscosity and mesophasegrowth, during processing, now exists.

[0088] While the invention has been shown in only one of its forms, itis not thus limited, but is susceptible to various changes andmodifications without departing the sprit thereof.

What is claimed is:
 1. A method of producing carbon foam, comprising thesteps of: introducing pitch to an appropriate level in a mold, the pitchhaving a characteristic boiling or foaming point at a given pressure andfor a given temperature; purging air from the mold; pressurizing thepitch between a preselected initial pressure and a final pressure, thepreselected initial pressure serving to increase the boiling or foamingpoint of the pitch above the foaming point at the final pressure;heating the pitch to a temperature below the solidification point butabove the liquid and foaming point which typically occurs at the finalpressure; depressurizing the pitch from the initial pressure to thefinal pressure while maintaining the process temperature above thetypical boiling or foaming temperature at the final pressure; heatingthe foamed pitch to a temperature that solidifies the foamed pitch;cooling the solidified foamed pitch to room temperature while allowingnatural depressurization during cooling to thereby produce a carbonfoam.
 2. The method of claim 1, wherein the pitch is introduced in aform selected from the group consisting of granulated pitches, powderedpitches and pelletized pitches.
 3. The method of claim 1, wherein thepitch is selected from the group consisting of mesophase and isotropicpitches and mixtures thereof.
 4. The method of claim 3, wherein thepitch is a mesophase pitch selected from the group consisting ofsynthetic pitches, coal based pitches, petroleum based pitches andmixtures thereof.
 5. The method of claim 1, wherein the mold is linedwith a metal liner.
 6. The method of claim 1, wherein the HIP ispressurized to an initial pressure between about 4000 and 16,000 psigwith an inert gas.
 7. The method of claim 6, wherein the inert gas isselected from the group consisting of nitrogen gas and argon gas.
 8. Themethod of claim 1, wherein the pitch is heated from room temperature toa melting temperature in the range from about 250 to 350° C. at a ratebetween about 1.0 and 10.0° C. per minute.
 9. The method of claim 1,wherein the pitch is held at a temperature between about 250 and 350° C.for up to about 10 hours.
 10. The method of claim 1, wherein the pitchis heated between about 250 and 450° C. at a rate between about 1.0 and10.0° C. per minute.
 11. The method of claim 1, wherein the pitch isheld between 250 and 450° C. from 0 to 4 hours.
 12. The method of claim1, wherein the HIP is depressurized from the initial pressure to thefinal pressure at a rate between 50 and 700 psig per minute whilemaintaining the pre-depressurization temperature.
 13. The method ofclaim 12, wherein the final pressure is between 50 and 4000 psig. 14.The method of claim 12, wherein, after depressurization, the foam isheated from its pre-depressurization temperature to a temperaturebetween 400 and 800° C. at a rate between 1.0 and 10.0° C. per minute.15. The method of claim 14, wherein the foam is held between 400 and800° C. from 0 to 4 hours.
 16. The method of claim 15, wherein the foamis cooled from between 400 and 800° C. to room temperature at a ratebetween 1.0 and 30° C. per minute.
 17. The method of claim 1, furthercomprising the steps of: heating the carbon foam in a non-oxidizingatmosphere to a temperature sufficient to carbonize the carbon foam toform a carbonized carbon foam; and heating the carbonized carbon foam ina non-oxidizing atmosphere to a temperature sufficient to form athermally conductive essentially graphitic carbon foam.
 18. The methodof claim 1, wherein the carbon foam is characterized as having a thermalconductivity ranging from about 90 to 244 W/mK.
 19. The method of claim1, wherein the carbon foam is characterized as having an apparentdensity ranging from about 0.43 to 0.62.
 20. The method of claim 1,wherein the carbon foam is characterized by the substantial absence ofinternal voids.
 21. A method of producing carbon foam, comprising thesteps of: selecting an appropriate mold shape; introducing pitch to anappropriate level in the mold, the pitch having a characteristic boilingor foaming point at a given pressure and for a given temperature;desiccating the pitch to assist in removing any residual moisture;placing the mold within a hot isostatic press and purging air from thepress and from the mold; pressurizing the pitch between a preselectedinitial pressure and a final pressure, the preselected initial pressureserving to increase the boiling or foaming point of the pitch above thefoaming point at the final pressure; heating the pitch to a temperaturebelow the solidification point but above the liquid and foaming pointwhich typically occurs at the final pressure; depressurizing the pitchfrom the initial pressure to the final pressure while maintaining theprocess temperature above the typical boiling or foaming temperature atthe final pressure; heating the foamed pitch to a temperature thatsolidifies the foamed pitch; cooling the solidified foamed pitch to roomtemperature while allowing natural depressurization during cooling tothereby produce a carbon foam; depressurizing the carbon foam toatmospheric pressure; and removing the carbon foam from the hotisostatic press and from the mold.